The field of the invention is solid state industrial control systems, and particularly, controllers formed by interconnecting solid state logic circuits which are functionally equivalent to relays.
Solid state controllers are often employed to control industrial processes and machines which have traditionally been controlled by relay networks, or panels. A practical limitation on their further use in such applications, however, has been the reluctance of control engineers and maintenance personnel to relinquish their knowhow and experience with relays in favor of Boolean algebra and logic circuit design. As a result, in recent years a number of solid state circuits have been proposed which simulate the operation of a relay and which can be combined with other similar circuits to form a solid state controller which simulates the operation of a relay controller.
In U.S. Pat. No. 3,774,051, for example, a time delay circuit is combined with a logic gate to provide a solid state equivalent of a relay. Similarly, U.S. Pat. No. 3,890,512 discloses a simulated relay comprised of interconnected inverter gates and AND gates. Although prior solid state relays simulate the logical functions of an electromagnetic relay, they do not simulate a number of electrical characteristics which are important in many applications. One such characteristic of relays not found in prior solid state simulations is the ability of the contacts to conduct current in either direction when they are closed. Also, when a relay coil controls a plurality of contacts, mechanical means are employed to insure that closed contacts open before open contacts close, or vice versa, when the coil is either energized or deenergized.